This invention relates to a process for the preparation of high-purity naphthalenecarboxylic acid esters in which a substituted naphthalene is oxidized with molecular oxygen in the presence of a heavy metal-based catalyst in a solvent comprising a lower aliphatic monocarboxylic acid to form a naphthalenecarboxylic acid (hereinafter abbreviated a NCA) and the NCA is then esterified to form an NCA ester.
Among NCA's, the monocarboxylic acid or naphthoic acid is useful in the production of photographic chemicals and dyestuffs. Naphthalene-dicarboxylic acids, and particularly naphthalene-2,6-dicarboxylic acid are useful in the production of various polyesters including polyethylene naphthalates and polyamides, which in turn are used in the manufacture of films and fibers having improved heat resistance, mechanical strength, and dimensional stability. Naphthalene-tricarboxylic acids and naphthalene-tetracarboxylic acids are promising as starting materials in the production of high-performance resins.
Various methods have been proposed for the preparation of NCA. A method which is widely used in the commercial production of NCA comprises an oxidation of an alkyl- or acyl-substituted naphthalene with molecular oxygen in the presence of a heavy metal-based catalyst in a solvent comprising a lower aliphatic monocarboxylic acid such as acetic acid.
The use of a catalyst comprising at least one heavy metal selected from cobalt and manganese and bromine in the above-mentioned method is disclosed in Japanese Patent Publication Nos. 48-43893(1973), 56-21017(1981), and 59-13495(1984), and Japanese Patent Application Kokai Nos. 49-42654(1974), 60-89445(1985), 60-89446(1985), 61-140540(1986), 61-246144(1986), and 63-104943(1988). Addition of an alkali metal to such a catalyst is proposed in Japanese Patent Application Kokai Nos. 61-246143(1986), 62-120343(1987), 62-120343(1987), 63-66150(1988), and 1-121240(1989).
The use of a catalyst comprising cobalt and nickel or cobalt and cerium as heavy metals and bromine is described in Japanese Patent Application Kokai Nos. 62-212343(1987) and 62-212344(1987).
NCA is frequently used in the form of its alkyl ester. Esterification of NCA is usually performed by reacting NCA with an alkyl alcohol in the presence of an esterification catalyst such as an acid including a Lewis acid. For example, it is proposed in Japanese Patent Publication No. 49-174(1974) that NCA is esterified with methyl alcohol in the presence of a catalyst selected from sulfuric acid, hydrogen chloride, hydrochloric acid, and organic sulfonic acids under pressure.
A crude ester product obtained by esterification of NCA is usually tinged with yellowish brown due to contamination with impurities. When such a crude ester product is used in the preparation of a polymer, the resulting polymer is undesirably colored and its quality is also degraded. Therefore, it is necessary to purify the crude ester product before use.
Various purification methods of crude NCA esters have been proposed. For example, recrystallization from hot methanol is described in Japanese Patent Application Kokai Nos. 50-84467(1975) and 50-111056(1975), while purification by distillation is described in Japanese Patent Application Kokai Nos. 57-35697(1982) and 58-29291(1983). Japanese Patent Application Kokai No. 50-116461(1975) discloses purification of a crude NCA ester by distillation followed by recrystallization. Similarly, Japanese Patent Application Kokai No. 117847/1989 discloses that a crude NCA ester is washed with methanol, distilled in vacuo, and finally recrystallized from hot methanol.
In order to prepare a high-purity NCA ester inexpensively on a commercial scale, it is important that the heavy metal catalyst used in the preparation of NCA be recovered and reused repeatedly.
Japanese Patent Publication No. 46-14339(1971) and Japanese Patent Application Kokai No. 47-34088(1972) disclose the preparation of terephthalic acid by liquid-phase oxidation of p-xylene in the presence of a heavy metal catalyst comprising cobalt and manganese. After terephthalic acid is collected from the reaction mixture by filtration, the filtrate is distilled to recover the solvent and the residue is treated so as to recover the heavy metals present therein as carbonates.
Japanese Patent Application Kokai No. 62-212345(1987) discloses a continuous process for the preparation of 2,6-naphthalene-dicarboxylic acid (2,6-NDCA) by oxidation of 2,6-diisopropylnaphthalene (2,6-DIPN) or its oxidation intermediate with molecular oxygen in the presence of a catalyst comprising cobalt, manganese, and bromine. The crude 2,6-NDCA crystals formed are separated and contacted with an aqueous mineral acid solution to dissolve out the heavy metal catalyst present thereon, and the resulting mineral acid solution from which 2,6-NDCA crystals have been separated is treated with an alkali metal carbonate or bicarbonate to recover the catalytic heavy metals as carbonates or bicarbonates.
Japanese Patent Application Kokai No. 1-121237(1989) discloses the purification of crude crystals of an aromatic carboxylic acid by washing with water or recrystallization and treating the washings or mother liquor of recrystallization with a carbonate ion-forming compound to precipitate heavy metals as carbonates.
Although various purification methods of NCA esters are known as described above, there is a need to prepare a high-purity, colorless NCA ester product efficiently and economically while recovering catalytic heavy metals for reuse.